Limiting device



Dec. 23, 1941. MATHEs 2,267,120

LIMITING DEVICE Original Filed March 18, 1939 K I INVENTOR. 0/0/ 220 5.m r/7w X BY r m ATTORNEY.-

Patented Dec. 23, 1941 5T: orgies LIMITING DEVICE" Richard E. Mathes,Westfield,; l \l. .L, assignon to; Radio'Oorporationof America,v acorporation of Delaware Original application March 18; 1939, Serial No.

262.664. Divided and this. application May '11-, l 1940-; Serial No.334,523

.2 Claims. (01. Lia-e44.) I

This invention relates to limiting devicesand:

is a division of my application filed March l8, 1939, Serial No.262,664.

It is an object of the invention to provide an improved limiter for boththe positive and the negative halves of an alternating wave.

Another object is to provide a limiter that symmetrically limits thepositive and negative halves of an alternating wave.

Another object is to provide a push-pull limiter using negative biascut-off on both halves of the wave.

Another object is to provide a full wave rectifier in combination with abias cut-01f limiter to eliminate all limiting that would be caused byflow of grid current.

Other objects will appear in the following description with referenceto, the drawing in which:

Fig. 1 is a circuit diagram of the improved limiter.

Fi 2 is a series of graphs illustrating the action of the limiter andthe associated rectifier.

In prior art constructions it is usual to rely on the blocking of a tubeby a negative grid bias for limiting the negative half wave and onelectron saturation or grid energy consumption for limiting the positivehalf wave. The negative and positive parts of the wave passed throughsuch prior art limiters are dissimilar and give rise to harmonics whichin many cases are objectionable. I overcome this by the limiter in Fig.1.

In Fig. 1 the current to be limited is fed into transformer I having twosecondary coils 2, 3 connected to the anodes 4 and 5 of the doublerectifier 6, which, of course, can be two separate rectifiers, ifdesired. The two coils 2, 3 are connected together by resistances 1 andB. The conductor joining these two resistances is grounded at 9. Theungrounded ends of resistances l, 8 are connected to grids ID, ll ofvacuum tubes l2, l3, respectively, in push-pull. The anodes l4 and [5 ofthese tubes are also connected in push-pull by transformer primaries l6,ll.

The cathodes of the double rectifier 6 are connected by adjustablecontact l8 to resistance l9, one terminal 20 of which is connected topositive potential and the other terminal 2! of which is a grounded at22. The cathodes of vacuum tubes [2, l3 are connected by adjustablecontact 23 with resistance 24, one terminal 25 being connected to groundand the other terminal 26 being connected to a positive source ofpotential.

The output of transformer primaries 16, ll

passesinto any desired device, indicated? generallyas:acoil.=21..

'I'heoperatiorr of my invention isas follows:

Letitbe supposed thatthe upper endxof'coil" 2 as shown in the drawingis. for the instant-made positive by: the incoming signal or otheralternating wave.-

coil 3 is negative. It also means that the lower end of coil: 2 isnegative in respect to the upper positive the wave passing throughrectifier 6 is indicated for the purposes of illustration by 28 ofgraphI' in Fig. 2. No current from the lowercoil 3 can passthrough therectifier 6 because the negative end is connected to-=theanode 5.Therefore no signal" potential will beappl-ied to grid l-I of tube 13.For the half wave of the signal under discussion the plate current Ip oftube l3 will be a steady current, as indicated at 29 of graph II in Fig.2. During this same half cycle the current Ip through the plate circuitof tube I2 is decreased by the negative potential applied to it byresistance 1, and therefore the graph of plate current in this tube isgiven by 30 of graph III. It will be seen that the current in this graphreaches the cut-off point at 3| and current flow through the tube ceasesat this point. The dotted line 32 indicates the wave shape if there hadbeen no cut-oil? of the current by the negative grid potential.

When the negative potential on grid H) decreases to the pointcorresponding to point 33 of the plate current curve, current starts toflow in the plate circuit of tube l2 and increases until it reaches itsmaximum value at 34, at which point the other half wave of the signalreverses the polarity in coils 2 and 3. For this half cycle the currentthrough the rectifier 6 will be shown at 35, which, of course, is thesame shape as the other half wave 28. No signal potential is applied tothe grid of tube l2 at this time because of the reversal of polarity andits plate current will be given by the straight portion between points34 and 36. At this time .a negative potential is applied to the grid oftube [3 and from the beginning of the half cycle indicated at point Thismeans that the lower end of 31 the negative potential increases and theplate currentI decreases until the cut-off point 38 is reached (graphII).

No current will then flow in the plate circuit of tube l3 until thenegative potential on grid l I decreases to the cut-off point indicatedby 39. As the negative potential on grid l3 continues to decrease untilthe half cycle is ended, the plate current in this tube will increasetill it reaches point 40. For the next half cycle the plate currentthrough tube l3 will be given by the straight line between points 40 andM.

Since the tubes l2 and I3 are arranged in push-pull 'fashion, it isobvious that graphs II and III will produce a combined output indicatedby graph IV of Fig. 2. This curve also indicates in general the inducedelectromotive force in output coil 21.

In the beginning it was assumed that slider I8 was at ground potential.If we assume that it is adjusted to place a positive voltage on thecathodes of rectifier 6, no current can flow through the rectifiersuntil the signal voltage reaches a threshold value in excess of thiscounter voltage. is then indicated generally by graph'V of Fig. 2, whereat the beginning and ending of each half Wave no current flows, asindicated at 4| This will be the shape of the rectified wave of negativepotential applied alternately to tubes l2. and I3 instead of the shapegiven in graph I. It will be seen that the shape of the current fiow orE. M. F., as the case may be, will be indicated in graph VI, which issimilar to graph IV except that there are appreciably lengths of timewhen current does not flow in the output circuit, as indicated at 42.The cut-ofi value in the rectifier 6 and the push-pull limiter tubes [2,l3 can be varied singly or jointly within any desired limits byadjustment of the resistance taps l8 and 23.

It will be seen that in my improved limiter the signal or otheralternating current entering at input 2, 3 will pass into the output 21as an alternating current with the tops of the waves cut offsymmetrically, thus producing symmetrical limiting.

The shape of the rectified current The limiter of my invention may beassociated with any type of input or output, as the invention is notlimited to any particular use.

Having described my invention, what I claim 1. In a voltage limiter, atransformer having two secondary coils, a resistance connecting saidcoils, a rectifier connected across one of said secondary coils and partof said resistance, a rectifier connected across the other of saidsecondary coils and another part of said resistance,

an amplifying tube having its control electrode termined limiting point,and means for connect-' ing the anodes of said tubes in opposition.

2. In a voltage limiter, a transformer having two secondary coils, aresistance connecting said coils, a rectifier connected across one ofsaid secondary coils and part of said resistance, a rectifier connectedacross the other of said secondary coils and another part of saidresistance, an amplifying tube having its control electrode and cathodeconnected respectively to negative and positive potential points in thefirst mentioned part of said resistance, an amplifying tube having itscontrol electrode and cathode connected respectively to negative andpositive potential points in the other part of said resistance, a sourceof negative bias between the cathodes and grids of said amplifier tubessufficient to bring the tubes to cut off at the predetermined limitingpoint, a threshold for said rectifiers comprising a source ofcounterelectromotive force in series therewith and means for connectingthe anodes of said tubes in opposition.

RICHARD E. MATHES.

